1. Field of the Invention
The present specification relates to a wireless communication, more particularly, to a method and an apparatus for transmitting a training field which is designed to demodulating a data field.
2. Related Art
Recently, Machine-to-Machine (M2M) system is spotlighted as the next generation communication technology, and new standard for it has been in the course of establishment in order to technically support it in IEEE 802.11 WLAN. The M2M system means the network in which machinery, as a main agent of communication, not human exchanges information. The components of M2M are widely ranged from sensors to detect temperature and humidity, electric home appliance such as camera, TV and etc. to large-scale machines such as machinery tools and automobiles. Recently, with emerging various communication services such as smart grid, eHealth and ubiquitous, M2M technology is widely used in order to technically support the communication services. The characteristics of M2M system are as follows.
1) Large number of station: As for M2M, large number of station is assumed that is different from the existing network. That is because sensors installed on house, company, and etc as well as machines of personal belongings should be considered. Accordingly, considerably large number of station may be connected to a single AP.
2) Low traffic load per each station: Because M2M terminal has traffic pattern which is collecting and reporting information around, it is not necessary to communicate data frequently but also the amount of data is not considerable.
3) Uplink centric: M2M has structure that reports the data using uplink after receiving commands and executing procedures. Since main data is commonly forwarded using uplink, consequently, uplink is mainly used in M2M.
4) Long durability of Station: M2M terminals are operated by mainly battery, and user frequently recharges the battery in many cases. Accordingly, long durability is demanded by minimizing consumption of the battery.
5) Automatic recovery function: M2M terminals are not operable by users in certain situation. Accordingly, automatic recovery function is necessary.